Can Elastomers Combine Stiffness, Toughness and Fatigue Resistance?

Have you ever tried to wrap a stack of cards with a new rubber band and then repeated the same task with an old one? While new rubber bands are elastic and can easily encircle the stack, older rubber bands tend to lose elasticity, become brittle, and ultimately rupture. Rubber is a sub-class of elastomers, which are solvent-free polymeric networks. Above their glass transition temperature, T_g, elastomers are characterized by a high stretchability. Yet, many homogeneous single network elastomers display a low mechanical strength and limited durability. Given their significance, for example for protective sport equipment, damping systems, and in soft robotics, extensive efforts have been dedicated to developing elastomers that combine stiffness, toughness, and durability. The ideal elastomer for soft robotics, wearables, and biomedical applications would possess spatially variable mechanical properties, enabling controlled deformation, sustaining considerable loads, efficiently damp energy, and be fatigue resistant. However, current elastomer formulations cannot simultaneously achieve all of these attributes. This review summarizes the current understanding of the structure-mechanical property relationship of elastomers and highlights recent strategies that overcome certain trade-offs of the mechanical properties of elastomers by locally varying their composition.